1. Field of the Invention
The present invention relates to an active-matrix display device including light-emitting elements in its pixels, and a method for driving the same. Furthermore, the present invention relates to electronic apparatus including such a display device.
2. Description of the Related Art
In recent years, development of flat self-luminous display devices employing organic EL (Electro Luminescence) devices as light-emitting elements is being actively promoted. The organic EL device is based on a phenomenon that an organic thin film emits light in response to application of an electric field thereto. The organic EL device can be driven by application voltage of 10 V or lower, and thus has low power consumption. Furthermore, because the organic EL device is a self-luminous element that emits light by itself, it does not need an illuminating unit and thus easily allows reduction in the weight and thickness of a display device. Moreover, the response speed of the organic EL device is as very high as about several microseconds, which causes no image lag in displaying of a moving image.
Among the flat self-luminous display devices employing the organic EL devices for the pixels, particularly an active-matrix display device in which thin film transistors are integrally formed as drive elements in the respective pixels is being actively developed. Active-matrix flat self-luminous display devices are disclosed in e.g. Japanese Patent Laid-Open Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791, and 2004-093682.
FIG. 28 is a schematic circuit diagram showing one example of an active-matrix display device of a related art. This display device includes a pixel array part 1 and a peripheral drive part. The drive part includes a signal driver 3 and a write scanner 4. The pixel array part 1 includes signal lines SL disposed along the columns and scan lines WS disposed along the rows. Pixels 2 are disposed at the respective intersections of the signal lines SL and the scan lines WS. FIG. 28 shows merely one pixel 2 for easy understanding. The write scanner 4 includes shift registers. The shift registers operate in response to a clock signal ck supplied from the external and sequentially transfer a start pulse sp supplied from the external similarly, to thereby output a control signal to the scan lines WS sequentially. The signal driver 3 supplies a video signal to the signal lines SL in matching with the line-sequential scanning by the write scanner 4.
The pixel 2 includes a sampling transistor T1, a drive transistor T2, a hold capacitor C1, and a light-emitting element EL. The drive transistor T2 is a P-channel transistor. The source thereof as one current terminal thereof is connected to a power supply line and the drain thereof as the other current terminal thereof is connected to the light-emitting element EL. The gate of the drive transistor T2 as the control terminal thereof is connected to the signal line SL via the sampling transistor T1. The sampling transistor T1 is turned on in response to the control signal supplied from the write scanner 4 to thereby sample the video signal supplied from the signal line SL and write it to the hold capacitor C1. The drive transistor T2 receives, at its gate, the video signal written to the hold capacitor C1 as a gate voltage Vgs, and causes a drain current Ids to flow to the light-emitting element EL. This causes the light-emitting element EL to emit light with the luminance dependent upon the video signal. The gate voltage Vgs refers to the potential of the gate relative to that of the source.
The drive transistor T2 operates in the saturation region, and the relationship between the gate voltage Vgs and the drain current Ids is represented by the following characteristic equation.Ids=(1/2)μ(W/L)Cox(Vgs−Vth)2 In this equation, μ denotes the mobility of the drive transistor, W denotes the channel width of the drive transistor, L denotes the channel length of the drive transistor, Cox denotes the capacitance of the gate insulating film of the drive transistor per unit area, and Vth denotes the threshold voltage of the drive transistor. As is apparent from this characteristic equation, the drive transistor T2 functions as a constant current source that supplies the drain current Ids depending on the gate voltage Vgs when it operates in the saturation region.
FIG. 29 is a graph showing the voltage-current characteristic of the light-emitting element EL. In this graph, an anode voltage V is plotted on the abscissa and the drive current Ids is plotted on the ordinate. The anode voltage of the light-emitting element EL is equivalent to the drain voltage of the drive transistor T2. The light-emitting element EL has a tendency that its current-voltage characteristic changes over time and the characteristic curve gradually falls down along with time elapse. Therefore, the anode voltage (drain voltage) V changes even if the drive current Ids is constant. However, in the pixel circuit 2 shown in FIG. 28, the drive transistor T2 operates in the saturation region and allows the flowing of the drive current Ids dependent upon the gate voltage Vgs irrespective of change in the drain voltage. This makes it possible to keep the light-emission luminance constant irrespective of aging change in the characteristic of the light-emitting element EL.
FIG. 30 is a circuit diagram showing another example of a related-art pixel circuit. This pixel circuit is different from the pixel circuit shown in FIG. 28 in that the drive transistor T2 is not a P-channel transistor but an N-channel transistor. In many cases, it is more advantageous that all of the transistors included in the pixel are N-channel transistors in terms of the circuit manufacturing process.
However, in the circuit configuration of FIG. 30, because the drive transistor T2 is an N-channel transistor, the drain thereof is connected to the power supply line and a source S thereof is connected to the anode of the light-emitting element EL. Therefore, if the characteristic of the light-emitting element EL changes over time, the potential of the source S is affected and thus Vgs changes, which leads to aging change in the drain current Ids supplied from the drive transistor T2. Thus, the luminance of the light-emitting element EL changes over time. Not merely the characteristic of the light-emitting element EL but also the threshold voltage Vth of the drive transistor T2 involves variation from pixel to pixel. Because the parameter Vth is included in the above-described transistor characteristic equation, Ids changes even if Vgs is constant. This results in variation in the light-emission luminance from pixel to pixel, which precludes achievement of the screen uniformity. As a related art, there has been proposed a display device having a function for correction against the variation in the threshold voltage Vth of the drive transistor T2 from pixel to pixel (threshold voltage correction function). This display device is disclosed in e.g. the above-mentioned Japanese Patent Laid-Open No. 2004-133240.
Besides the threshold voltage Vth, the mobility μ of the drive transistor T2 also involves variation from pixel to pixel. Because the parameter μ is included in the above-described transistor characteristic equation, Ids changes even if Vgs is constant. This results in variation in the light-emission luminance from pixel to pixel, which precludes achievement of the screen uniformity. As a related art, there has also been proposed a display device having a function for correction against the variation in the mobility μ of the drive transistor T2 from pixel to pixel (mobility correction function).
In the mobility correction, the optimum correction amount and correction time differ depending on the level of the video signal to be written to the pixel (luminance). It is difficult for the display device having the related-art mobility correction function to achieve the optimum mobility correction amount dependent upon the luminance, which is a problem that should be solved. Furthermore, it is difficult for the related-art mobility correction function to achieve the accurate and proper correction amount due to the influence of fluctuation in the potential of the signal line, which is a problem that should be solved.